The proposed research is designed to explore the origin, genealogy and mode of evolution of selected bacterial pathogens. Using the techniques of cloning and DNA sequencing, comparative analysis of the same gene in several different species can be used to determine genealogical relationships and relative evolutionary rates. The molecule of choice for determining bacterial relationships is the 16S ribosomal RNA; which is found ubiquitously, and for which there now exists a considerable database for comparisons. The genera to be studied include various human pathogens: Rickettsia, Chlamydia, Treponema, Mycoplasma, and Bacteroides. DNA from these organisms will be cloned in Lambda phage, subcloned into M13 bacteriophage, and sequenced with primers specific for common regions of the 16S rRNA sequence. The objective of the proposed research is to determine the genealogical relationships of some obligate and facultative pathogens and explore their mode of evolution -- i.e. to ask whether the evolution of pathogenesis has any special characteristics. The knowledge of genetic relatedness among and between these taxa bears directly on resistance gene transfer, development of optimal systems for the study of various diseases, host-vector systems for making vaccine antigens, potential new diseases, and new approaches to the study of these pathogenic bacteria.